1. Technical Field
The present invention generally relates to a light source apparatus having separate light emitting devices for red light, green light, and blue light, and an image projection apparatus employing the light source apparatus.
2. Background Art
Image projection apparatuses include various types of projectors such as a digital light processing (DLP: registered trademark) projector, a liquid crystal display (LCD) projector, and so on. The projector may use a discharge lamp such as a high-pressure mercury vapor lamp as a light source, but a semiconductor light source or a solid-state light source such as a light emitting diode (LED) or a laser diode (LD) has been developed and can be used instead of the discharge lamp. For example, JP-2012-008549-A discloses an image display apparatus employing an LED as the light source. The light intensity of the semiconductor light sources has been enhanced compared to the previous types of semiconductor light sources. Further, the semiconductor light sources have environmentally preferable features such as mercury-free construction and low power consumption.
The high-pressure mercury vapor lamp has a specific emission spectrum characteristic such that the blue (B) component is strong while the green (G) and red (R) components are weak, by which the lamp emits light with a specific color reproduction performance. The Commission Internationale de l'Eclairage (CIE) defines an international color system such as CIE-XYZ color system, such that the color coordinates can be set in the xy chromaticity diagram, and sRGB (standard RGB) is used as the international Standard, in which a color triangle is defined by connecting the tops of R, G and B defined by sRGB.
Compared to the color triangle of sRGB, the green light of the high-pressure mercury vapor lamp is shifted toward yellowish green, and the red light of the high-pressure mercury vapor lamp is shifted toward vermilion, as shown in FIG. 6. Therefore, to secure the intensity of green light and red light for the projector using the high-pressure mercury vapor lamp, a light having a wavelength deviated from the wavelength of the primary colors defined by sRGB standard is used as primary color light. As a consequence, however, the color reproduction performance of the projector using the high-pressure mercury vapor lamp deteriorates.
In contrast, projectors employing the semiconductor light sources can use separate red, green, and blue light sources, and each of the light sources can be configured to emit a light beam having a specific wavelength. Therefore, compared to projectors using a high-pressure mercury vapor lamp as the light source, the wavelengths of light beams emitted from each of the light source, used as the primary color, can be more closely matched to the wavelength of the primary color light as defined by sRGB standard. Therefore, a color triangle of the semiconductor light sources on the color coordinates of the xy chromaticity diagram can be set closer to the sRGB standard color triangle, thereby improving color reproduction performance and enlarging a color reproduction area. For this reason, the light sources for projectors may be changed from the high-pressure mercury vapor lamp to the semiconductor light source in the future. It should be noted that herein the term “primary colors” mean colors that become the sources of all colors, wherein the primary colors of light are red (R), green (G), and blue (B) light, and almost any color can be reproduced using some combination of the three primary colors.
As for the projectors using the semiconductor light sources, the color triangle defined by the wavelength of each of the light sources used as the primary colors can be set close to the color triangle of sRGB standard on the color coordinates, thereby providing color reproduction performance superior to projectors using the above-mentioned discharge lamp such as the high-pressure mercury vapor lamp. However, the color reproduction performance of non-primary colors generated by a combination of light of each of the light sources may not always be satisfactory.
In JP-2012-008549-A, for example, two light sources are used for the blue light, in which one light source such as a semiconductor laser emits a first blue light, and another light source such as a blue light generator emits a second blue light. The first blue light of the semiconductor laser has a wavelength of 450 nm or less, which is a monochromatic light having a relatively violet tone. The second blue light generated by the blue light generator is combined with the first blue light to broaden a wavelength range of the blue light to a long wavelength side so that color rendering property of the blue light can be enhanced.
However, in the configuration of JP-2012-008549-A, the color reproduction performance of non-primary color may not be satisfactory, in particular for Ye (yellow) color. The color rendering property means a property of light of the light source that affects the color appearance of an object when the light emitted from the light source apparatus irradiates the object. Specifically, if the color appearance of the object irradiated by a specific light is close to the object's natural appearance in sunlight (reference light), the color rendering is said to be good, and if the color appearance of the object using the specific light is not natural, the color rendering is said to be not good.